Recently, in sintering machines, the supply of hematite and other iron ore used as the mainstream in the past has decreased, while the supply of iron ore with a high water of crystallization content (3 mass % or more) has increased. This iron ore of a high water of crystallization content has a great amount of fine powder compared to the iron ore used in the past, so when charging this iron ore into a sintering machine without pretreatment, the ventilation of the sintering machine is inhibited and it is not possible to productively produce sintered ore of a good quality.
Consequently, it is necessary to granulate the iron ore before charging it into the sintering machine, but there are the defects that the wettability with water is poor and the granulatability is low compared to the iron ore used in the past, so technology to granulate this has become necessary.
Usually, as a granulation technology, the method of making the fine powder stick to the coarse grains forming core grains (the granules formed by this method being referred to below as the “S-type granules”) has been the mainstream, but the method of granulating only the fine powder or mainly the fine powder (the granules formed by this method being referred to below as the “P-type granules”) has also been proposed.
For example, Japanese Patent Publication (A) No. 4-80327 discloses the technology of pulverizing iron ore and limestone so that the grains of 250 μm or less become 80 wt % or more and producing P-type granules in the presence of water. Further, Japanese Patent Publication (A) No. 53-123303 discloses the technology of granulating granules of iron ore two times to produce granules.
However, in the above conventional methods for pretreating sintering materials, there were the following problems which still should be solved.
The method disclosed in Japanese Patent Publication (A) No. 4-80327 requires that all of the limestone functioning as a binder be pulverized. This invites an increase of the production costs due to the pulverization and is not economical. The productivity of the granules is also extremely poor.
Further, with just making the pulverized grains of a size of 250 μm or less 80 wt % or more, it is not possible to raise the strength of the P-type granules produced up to the targeted strength. For example, when conveying the granules via a plurality of belt conveyors, the granules were liable to become powderized at the time of transfer.
The method disclosed in Japanese Patent Publication (A) No. 53-123303 may be able to improve the strength of the granules. However, for example, when preparing S-type granules, it is not possible to control the stuck thickness of the fine powder.
Consequently, if the stuck thickness is thick, the coke is buried inside the granules and it is difficult to produce a sintered ore providing the desired quality. This invites a drop in yield of the sintered ore and impairs the productivity of the sintered ore.